1. Field of the Invention
The present invention relates to an image forming apparatus, which is to be used in electrophotographic machines, such as printers, copiers and facsimiles, for forming a full-color image as the individual color images formed by plural electrostatic recording unite are overlapped. Also, the invention relates to a recording medium in which a test-pattern image forming program is recorded, a test-pattern image forming method, and a skew angle calculating method.
2. Description of the Related Art
As electrophotographic color printers, a tandem type is hitherto known in which a plurality of electrostatic recording units (image forming units) are arranged in sequence in the direction of conveyance of recording paper.
The typical tandem color printer is equipped with four electrostatic recording units for four colors, i.e. black (K), cyan (C), magenta (M) and yellow (Y). Each of the four electrostatic recording units forms a latent image by optically scanning a photosensitive drum based on image data, and then develops the latent image with color toner of the individual color in a developing device. After having been developed with color toner of the individual colors on these four electrostatic recording units, the four-color toner images are transferred onto the recording paper, which is being conveyed at a constant speed, as overlapped one over another in the order of yellow (Y), magenta (M), cyan (C) and black (K). Subsequently this composite image is fixed on the recording paper by a fixing device to complete a full-color print.
For improving the quality of color print in such tandem-type color printer, it is necessary to minimize positional errors (print errors) of the toner images, which are to be transferred onto the recording paper by the individual electrostatic recording units, to increase the precision of color registration.
For example, if the solution in either of the primary scanning direction (perpendicular to the direction of conveyance of the recording paper) and the secondary scanning direction (the direction of conveyance of the recording paper) on the recording paper is 600 dpi (dots per inch), the inter-dot pitch is approximately 42 xcexcm; the positional errors of the toner images must be limited to the pixel pitch within this range.
Consequently it has been customary to previously detect amounts of errors in a primary scanning direction, a secondary scanning direction and an oblique direction (inclined with respect to the direction of conveyance of the recording paper) and then to correct these positional errors.
As one example, image data corrected in accordance with the individual amounts of positional errors is expanded on a bitmap memory (image memory). And as another example, the timing to read out the expanded image data from the bitmap memory. As still another example, the error of position in the secondary scanning direction in particular is corrected by varying the timing of exposure by an exposure device.
For realizing effective utilization of positional error correction to restrict the positional error extents within the range of less than approximately 42 xcexcm in pixel pitch, it is necessary to exchange parts and adjust mechanical positions with precision prior to shipping from a factory or when maintenance, inspection or repairing by a custom engineer (CE) after installation of a product printer at user""s site.
For example, in the presence of a problem in accuracy of manufacture, such as when an exposure device equipped with the individual electrostatic recording unit curves in the scanning direction, when the inter-dot pitch of the exposure device is poor, or when the axis of a photosensitive drum is off the center line, it is hard to correct the positional errors. As that is the case, it would then be more important to exchange parts and adjust mechanical positions so that the positional errors can be within a predetermined allowance by the above-mentioned correction of position error amounts.
Consequently, when shipping from a factory, for example, it is necessary to grasp a cause for positional error by printing a test-pattern image to check what kind of the positional error has occurred in a printer to be shipped. Then, the position is corrected mechanically or the involved part is exchanged with a new one to provide an allowance of positional errors will cope with by the above-mentioned correction.
However, since possible positional errors with color printers include those which would occur with lapse of time, irrespective of the accuracy of manufacture of various parts, it is necessary to grasp causes of positional errors by printing a test-pattern image also at the time of maintenance, inspection and repairing by a custom engineer (CE) likewise at the time of the above-mentioned shipping at a factory and observing the positional errors. And then it is necessary to mechanically correct the positions and/or to exchange of the parts. Thus it has long been cherished to perform the necessary procedure efficiently without expert""s labor.
Even if such test-pattern image is printed, it yet would take time to discriminate what kinds of causes for the positional errors. Thus, after printing a test-pattern image, the operator ensures whether a positional error or errors have occurred from the printed test-pattern image. The operator then makes positional adjustments and exchange with respect to the parts that are presumably causes for the positional errors, whereupon the operator makes a print of a test-pattern image again and ensures whether the positional adjustments have been made with precision. If the positional errors have not been corrected as this result, it has been a common practice to repeat positional adjustment and/or exchange of other parts, which would be time-consuming and need expert""s labor.
One solution technology has been proposed which is accomplished by grasping the positional errors using a variety of test-pattern images and discriminating causes for the errors.
Specifically, as the test-pattern images, {circle around (1)} a single line is printed in black (K) and three other lines are printed in parallel to the black line in yellow (Y), magenta (M) and cyan (C), or {circle around (2)} the yellow (Y), magenta (M) and cyan (C) lines are printed so as to extend beyond the black (K) line, or {circle around (3)} the yellow (Y), magenta (M) and cyan (C) are printed so as to cross the black (K) line to form a two-color cross mark.
In this conventional technology, since the test-pattern image printed for grasping the positional errors has to identified using a microscope or a magnifying glass, it is very meticulous so that efficient jobs would hardly been achieved.
As an alternative solution, Japanese Patent Laid-Open Publication No. HEI 9-304992 discloses a technology in which a test-pattern image composed of longitudinal or transverse lines are printed in fragments on recording paper, and printing errors in the primary, secondary and oblique scanning directions are detected for information enabling the operator to make adjustments without difficulty.
According to this conventional technology, although the extent of a positional error can be recognized by eyes, it is impossible to grasp a continuous change of the positional error as the test-pattern image is arranged only in fragments on recording-paper. Further, when non-uniform printing occurs due to the difference in extent of positional error between local positions of the recording paper, it is impossible to grasp the cycle and amplitude of the non-uniformity of printing. As a result, it is difficult to judge the cause for the positional error exactly.
As still another alternative solution, Japanese Patent Laid-Open Publication No. HEI 10-115955 discloses a technology which comprises previously providing a memory with a test-pattern image composed of plural groups of lines extending parallel to the primary scanning direction, printing an overlapped test-pattern image of a reference color (magenta, for example) and another color (yellow, for example), and estimating a positional error due to the inclination with respect to the primary scanning direction in terms of moire stripes that inevitably occur with the positional error due to the inclination (oblique print divergence, or skew) with respect to the lines extending in the primary scanning direction.
In this conventional technology, however, although the skew angle can be measured only in accordance with the number of moirxc3xa9 stripes, it is difficult to grasp the cause for the positional error in the presence of a curvature in the scanning direction or if the inter-dot pitch accuracy is poor. It is also difficult to grasp the direction of skew, namely, whether the positional error due to the inclination of lines, which extend in the primary scanning direction, with respect to the secondary scanning direction is a rising-on-the-right positional error or a rising-on-the-left positional error.
With the foregoing problems in view, it is a first object of the present invention to provide an image forming apparatus which enables exchange and positional adjustment of parts quickly and efficiently by easily grasping causes for possible positional errors using a simple test pattern, in maintenance, inspection and repairing of the apparatus by a custom engineer (CE) before shipping from a factory or after installation at the user""s site.
A second object of the invention is to provide a recording medium in which a test-pattern forming program is stored for use in printing a test-pattern image on the image forming apparatus for the above-mentioned purpose.
A third object of the invention is to provide a method for printing the test-pattern image on the image forming apparatus for the above-mentioned purpose.
A fourth object of the invention is to provide a method for calculating a skew angle (positional error) using the test-pattern image as printed on the image forming apparatus for the above-mentioned purpose.
According to a first generic feature of the present invention, there is provided an image forming apparatus comprising: a plurality of image forming units for forming different color images on recording paper, the plural image forming units including a first color image forming unit for forming a first color image, and at least one second color image forming unit for forming a second color image; and test-pattern image forming control means for controlling the first and second image forming units to form a test-pattern image composed of first and second marks different in density, which are arranged adjacent to one another, based on test-pattern image data that includes first data serving to arrange a plurality of first color lines, each having a predetermined line width, at a predetermined pitch by the first color image forming unit, second data serving to form the first mark by arranging a plurality of second color lines, each having a line width equal to that of the individual first color line, at a pitch equal to that of the first color lines by the at least one second color image forming unit so as to overlap with the first color lines within a first region occupying part of one area where of the plural first color lines are arranged, and third data serving to form the second mark by displacing a plurality of second color lines in a direction perpendicular to the first color lines within a second region contiguous to the first region and by arranging the second color lines, each having a line width equal to that of the individual first color line, at a pitch equal to that of the first color lines by the at least one second color image forming unit.
With this image forming apparatus, it is possible to form a test-pattern image with a simple construction and also to recognize a positional error easily by eyes, facilitating maintenance, inspection, repairing by the customer engineer at user""s site or inspection before shipping the apparatus from a factory.
As a preferable feature: the second region is composed of a plurality of sub-regions; the third data is data serving to form a plurality of second marks, which are different in density from one another, by varying the extent of displacement of the second color lines from the first color lines in the plural sub-regions of the second region stepwise; and the test-pattern image forming control means controls the first and second image forming units to form the test-pattern image such that the first mark and the plural second marks are disposed adjacent to one another as they are different in density from one another.
With this preferable feature, partly since the second region is divided into a plurality of sub-regions, and partly since a plurality of second marks different in density are formed by arranging the second color lines in the sub-regions with varying the extents of displacement with respect to the first color lines stepwise, it is possible to recognize the positional error more easily.
As another preferable feature: the predetermined line width is equal to the size of a single dot; the second region is composed of a plurality of sub-regions; the third data is data serving to form a plurality of second marks of different densities by varying the extent of displacement of the second color lines from the first color lines in the plural sub-regions of the second region stepwise by one dot for every sub-region; and the test-pattern image forming control means controls the first and second image forming units to form the test-pattern image such that the first mark and the plural second marks are disposed adjacent to one another as they are different in density from one another.
With this second preferable feature, partly since each of the first and second color lines has a predetermined line width corresponding to the size of a single dot, and partly. since the second marks different in density are formed in the plural sub-regions of the second region by arranging the second color liens with varying the extents of displacement with respect to the black lines stepwise by one dot for every sub-region, it is possible to judge a positional error in the order of 1 dot easily by eyes.
As still another preferable feature: the first image forming unit is a black-dedicated image forming unit for forming black lines as the first color lines; the at least one second image forming unit includes a magenta-dedicated image forming unit for forming magenta lines as the second color lines, a cyan-dedicated image forming unit for forming cyan lines as the second color lines, and a yellow-dedicated image forming unit for forming yellow lines as the second color lines; and the test-pattern image forming control means controls the black-, magenta-, cyan- and yellow-dedicated image forming units in such a manner that at least one of a cyan test-pattern image of the black and cyan lines, a magenta test-pattern image of the black and magenta lines, and a yellow test-pattern image of the black and yellow lines, is formed as the test-pattern image.
With the third preferable feature, by forming only the test-pattern image needed for grasping the occurrence of a positional error, it is possible to recognize the positional error efficiently. Further, since the test-pattern image is formed with selecting black, which is most contractive, as a reference color, it is possible to recognize a positional error more easily.
As a further preferable feature: each of the first and second color lines is a transverse line extending in a primary scanning direction perpendicular to a direction of conveyance of the recording paper; and the test-pattern image composed of the first and second marks contains a longitudinal succession of portions of the plural first color lines extending longitudinally and is formed on the recording paper at at least scanning-start- and scanning-end-side marginal regions in a secondary scanning direction perpendicular to the primary scanning direction.
With this fourth preferable feature, since the test-pattern image is formed in the above-mentioned manner, it is possible to realize the following judgments, using this test-pattern image when the printer is shipped from a factory or when the custom engineer repairs or inspects the printer in user""s site. It is accordingly possible to grasp a change in the secondary scanning direction in extent of positional error in the secondary scanning direction by observing this test-pattern image by eyes, thereby discriminating whether or not there have occurred any change of rotational speed of the individual photosensitive drum or any change of feed speed of recording paper (print sheet), which can be assumed as a cause for the change in the secondary scanning direction in extent of positional error in the secondary scanning direction (uneven printing in the secondary scanning direction).
As an additional preferable feature: each of the first and second color lines is a transverse line extending in a primary scanning direction perpendicular to a direction of conveyance of the recording paper; and the test-pattern image composed of the first and second marks contains a longitudinal succession of portions of the plural first color lines and is formed on the recording paper continuously from a scanning-start-side marginal region to a scanning-end-side marginal region in a secondary scanning direction perpendicular to the primary scanning direction.
With this fifth preferable feature, since the test-pattern image is formed on the recording paper continuously from the scanning-start-side to the scanning-end-side in the secondary scanning direction, it is possible to recognize a change in the second scanning direction in positional error extent in the secondary scanning direction more precisely.
As another preferable feature: each of the plural image forming units is an electrostatic recording unit equipped with a photosensitive drum; each of the first and second color lines is a transverse line extending in a primary scanning direction perpendicular to a direction of conveyance of the recording paper; and the test-pattern image composed of the first and second marks contains a longitudinal succession of portions of the plural first color lines and is formed on the recording paper so as to extend longitudinally by a length longer than a circumferential length of the photosensitive drum associated with the respective one of the first and second image forming units.
With this sixth preferable feature, since the test-pattern image is formed so as to extend in the secondary scanning direction by a length larger than the circumferential length of the photosensitive drum associated with a respective one of the first and second image forming units, it is possible to recognize precisely a change of rotational speed of the photosensitive drum which change would presumably be a cause for the change in the secondary scanning direction in the positional error extent in the secondary scanning direction.
As still another preferable feature: each of the first and second color lines is a longitudinal line extending in a direction of conveyance of the recording paper; and the test-pattern image composed of the first and second marks contains a transverse succession of portions of the plural first color lines extending longitudinally and is formed on the recording paper at at least scanning-start- and scanning-end-side marginal regions in the primary scanning direction.
With this seventh preferable feature, by observing the test-pattern image, which is image forming apparatus and the modified test-pattern image forming method of the second embodiment, it is possible to judge a change in the primary scanning direction in the positional error extent in the primary scanning direction precisely in terms of the positional error in the secondary scanning direction of the individual mark as the test-pattern image with the second color lines TL2 displaced in the primary scanning direction is observed by eyes for shipment of the printer at a factory or for maintenance, inspection or repairing of the printer at user""s site by the customer engineer. As a result, it is possible to grasp a change in the primary scanning direction in extent of positional error in the primary scanning direction so that the customer engineer can discriminate whether or not there have occurred a staggering the primary scanning direction, it is possible to judge a change pitch of the exposure device or an positioning error of the exposure device, which can be assumed as a cause for the change in the primary scanning direction in extent of positional error in the primary scanning direction (uneven printing in the primary scanning direction). Then the customer engineer performs exchange of parts and mechanical adjustments of positions of parts exactly and efficiently.
As a further preferable feature: each of the first and second color lines is a longitudinal line extending in a direction of conveyance of the recording paper; and the test-pattern image composed of the first and second marks contains a transverse succession of portions of the plural first color lines extending longitudinally and is formed on the recording paper continuously from a scanning-start-side marginal region to a scanning-end-side marginal region in the primary scanning direction.
With this eighth preferable feature, by forming the test-pattern image on the recording paper continuously from the scanning-start-side to the scanning-end-side in the primary scanning direction perpendicular to the recording paper conveyance direction, it is possible to recognize a change in the primary scanning direction in positional error in the primary scanning direction with increased accuracy.
As an additional preferable feature: each of the first and second color lines is a longitudinal line extending in a direction of conveyance of the recording paper; and the test-pattern image composed of the first and second marks contains a transverse succession of portions of the plural first color lines extending longitudinally and is formed on the recording paper continuously from a scanning-start-side marginal region to a scanning-end-side marginal region in the secondary scanning direction.
With this ninth preferable feature, by observing the test-pattern image, which is formed with the second color lines displaced in the primary scanning direction, by eyes, it is possible to judge a change in the secondary scanning direction in positional error extent in the primary scanning direction in terms of positional error of the individual marks in the primary scanning direction. As the result, it is possible to grasp a change in the secondary scanning direction in extent of positional error in the primary scanning direction so that the customer engineer can discriminate whether or not there have occurred a staggering movement of the conveyor belt or an inclined posture of the drive-gear-attachment flange associated with the photosensitive drum, which would presumably a cause for the change in the secondary scanning direction (uneven printing in the secondary scanning direction) in positional error extent in the primary scanning direction. And the customer engineer can perform exchange of parts and mechanical adjustments of positions of parts exactly and efficiently.
As another additional preferable feature: each of the plural image forming units is an electrostatic recording unit equipped with a photosensitive drum; each of the first and second color lines is a longitudinal line-extending in a direction of conveyance of the recording paper; and the test-pattern image composed of the first and second marks contains a transverse succession of portions of the plural first color lines extending longitudinally and is formed so as to extend longitudinally by a length longer than a circumferential length of the photosensitive drum associated with the respective one of the first and second image forming nits.
With this tenth preferable feature, since the test-pattern image is formed so as to extend in the secondary scanning direction longer than the circumferential length of the photosensitive drum associated with a respective one of the first and second image forming units, it is possible to surely recognize the occurrence of inclination of the drive-gear-attachment flange of the photosensitive drum, which inclination would presumably be a cause for the change in the secondary scanning direction in positional error extent in the primary scanning direction.
As a further preferable feature: the first image forming unit is a black-dedicated image forming unit for forming black lines as the first color lines; the at least one second image forming unit is composed of a magenta-dedicated image forming unit for forming magenta lines as the second color lines, a cyan-dedicated image forming unit for forming cyan lines as the second color lines, and a yellow-dedicated image forming unit for forming yellow lines as the second color lines; and the test-pattern image forming control means controls the black-, magenta-, cyan- and yellow-dedicated image forming units in such a manner that a cyan test-pattern image of the black and cyan lines, a magenta test-pattern image of the black and magenta lines, and a yellow test-pattern image of the black and yellow lines, are formed in a series arrangement in the primary scanning direction as a single test-pattern image combination.
With this eleventh preferable feature, since black, which is most contrastive with the whole recording paper, is selected as a reference color of the test-pattern image, the customer engineer can grasps the positional error more easily.
As a still further preferable feature: each of the first and second color lines is a transverse line extending in a primary scanning direction perpendicular to a direction of conveyance of the recording paper; and the test-pattern image composed of the first and second marks contains a longitudinal succession of portions of the plural first color lines extending transversely and is formed on the recording paper continuously from a scanning-start-side marginal region to a scanning-end-side marginal region in the primary scanning direction.
With this twelfth preferable feature, by observing the test-pattern image, which is formed with the second color lines displaced in the secondary scanning direction, by eyes, the customer engineer can recognize a change in the primary scanning direction in positional error extent in the secondary scanning direction in terms of the positional error of the individual marks in the secondary scanning direction. As the result, it is possible to grasp a change in the primary scanning direction in extent of positional error in the secondary scanning direction so that the customer engineer can discriminate whether or not there have occurred non-parallel or curved scanning lines of the exposure device, which would presumably a cause for the change in the primary scanning direction in extent of positional error in the secondary scanning direction. And the customer engineer can perform exchange of parts and mechanical adjustments of positions of parts exactly and efficiently.
As another preferable feature: each of the first and second color lines is a transverse line extending in a primary scanning direction perpendicular to a direction of conveyance of the recording paper; and the test-pattern image composed of the first and second marks contains a longitudinal succession of portions of the plural first color lines extending transversely and is formed continuously from a scanning-start-side marginal region to a scanning-end-side marginal region in the primary scanning direction.
With this thirteenth preferable feature, since the test-pattern image is formed on the recording paper continuously from the scanning-start-side to the scanning-end-side in the primary scanning direction perpendicular to the recording paper conveyance direction, it is possible to recognize a change in the primary scanning direction in positional error in the secondary scanning direction with increased accuracy.
As still another preferable feature: the first image forming unit is a black-dedicated image forming unit for forming black lines as the first color lines; the at least one second image forming unit is composed of a magenta-dedicated image forming unit for forming magenta lines as the second color lines, a cyan-dedicated image-forming unit for forming cyan lines as the second color lines, and a yellow-dedicated image forming unit for forming yellow lines as the second color lines; and the test-pattern image forming control means controls the black-, magenta-, cyan- and yellow-dedicated image forming units in such a manner that a cyan test-pattern image of the black and cyan lines, a magenta test-pattern image of the black and magenta lines, and a yellow test-pattern image of the black and yellow lines, are formed in a series arrangement in the secondary scanning direction as a single test-pattern image combination.
With this fourteenth preferable feature, it is possible to recognize a positional error for every color efficiently. Further, since the test-pattern image is formed with black, which is most contrastive color, as a reference color, the customer engineer can recognize the positional error more easily.
As an additional preferable feature, the test-pattern image is a composite form of a plurality of pattern images to be formed on a single sheet of the recording paper, the plural pattern images including: a first pattern image in which each of the first and second color lines is a transverse line extending in a primary scanning direction perpendicular to the direction of conveyance of the recording paper, the first pattern image being composed of the first and second marks, containing a longitudinal succession of portions of the plural first color lines and being formed on the recording paper at at least a scanning-start-side marginal region and a scanning-end-side marginal region in a secondary scanning direction perpendicular to the primary scanning direction; a second pattern image in which each of the first and second color lines is a longitudinal line extending in the direction of conveyance of the recording paper, the second pattern image being composed of the first and second marks, containing a transverse succession of portions of the plural first color lines and being formed on the recording paper at at least a scanning-start-side marginal region and a scanning-end-side marginal region in the primary scanning direction; and a third pattern image in which each of the first and second color lines is a longitudinal line extending in the direction of conveyance of the recording paper, the third pattern image being composed of the first and second marks, containing a transverse succession of portions of the plural first color lines and being formed on the recording paper continuously from a scanning-start-side marginal region to a scanning-end-side marginal region in the secondary scanning direction.
With this fifteenth preferable feature, given that a plurality of test-pattern images are collectively printed on a single sheet of recording paper, the customer engineer can concurrently judge more than one cause for positional errors when shipping the printer from a factory or when repairing and inspection of the printer at user""s site. For the same reason, it is possible to judge more than one cause for position errors efficiently, without spending recording paper and toner more than necessary.
According to a second generic feature of the present invention, there is provided a recording medium in which a test-pattern image forming program, for instructing a computer to control a plurality of image forming units so as to form of a test-pattern image of different color images on recording paper, is stored, wherein the program instructs the computer to function as control means for controlling the first and second image forming units based on test-pattern image data that includes: first data serving to arrange a plurality of first color lines, each having a predetermined line width, at a predetermined pitch by a first color image forming unit of the plural color image forming units; second data serving to form the first mark by arranging a plurality of second color lines, each having a line width equal to that of the individual first color line, at a pitch equal to that of the first color lines by at least one second color image forming unit of the plural color image forming units so as to overlap with the first color lines within a first region occupying part of an area where the plural first color lines are arranged; and third data serving to form the second mark by displacing a plurality of second color lines in a direction perpendicular to the first color lines within a second region contiguous to the first region and by arranging the second color lines, each having a line width equal to that of the individual first color line, at a pitch equal to that of the first color lines by the at least one second color image forming unit.
With this test-pattern image forming program storing recording medium, by installing the above-mentioned test-pattern image forming program and test-pattern data from the recording medium before shipping the apparatus from a factory or when maintenance, inspection or repairing of the apparatus at user""s site, the customer engineer can form the test-pattern image in a simple fashion and hence recognize a positional error by eyes even if the image storage device of the apparatus does not previously have the program and the test-pattern data.
According to a third generic feature of the present invention, there is provided a test-pattern image forming method comprising the steps of: creating test-pattern image data that includes first data serving to arrange a plurality of first color lines, each having a predetermined line width, at a predetermined pitch, second data serving to form the first mark by arranging a plurality of second color lines, each having a line width equal to that of the individual first color line, at a pitch equal to that of the first color lines so as to overlap with the first color lines within a first region occupying part of an area where the plural first color lines are arranged, and third data serving to form the second mark by displacing a plurality of second color lines in a direction perpendicular to the first color lines within a second region contiguous to the first region and by arranging the second color lines, each having a line width equal to that of the individual first color line, at a pitch equal to that of the first color lines; forming a test-pattern image, which is composed of the first and second marks of different densities, by forming the first mark such that the second color lines overlap with the first color lines, based on the first and second data, and by forming the second mark in such a manner that the second color lines are displaced from the first color lines and are disposed adjacent thereto, based on the first, second and third data.
According to the test-pattern image forming method of the present invention, it is possible form the above-mentioned test-pattern image in a simple manner and also to recognize a positional error easily by eyes before shipping the apparatus from a factory or when maintenance, inspection or repairing by the customer engineer at user""s site.
According to a fourth generic feature of the present invention, there is provided a skew angle calculation method comprising the steps of: forming a test-pattern image composed of first and second marks different in density, which are arranged adjacent to one another, the first mark being formed by arranging a plurality of first color lines, each having a predetermined line width, at a predetermined pitch, and arranging a plurality of second color lines, each having a line width equal to that of the individual first color line, at a pitch equal to that of the first color lines so as to overlap with the first color lines within a first region occupying part of an area where the plural first color lines are arranged, the second mark being formed by displacing a plurality of second color lines in a direction perpendicular to the first color lines within a second region contiguous to the first region, and by arranging the second color lines, each having a line width equal to that of the individual first color line, at a pitch equal to that of the first color lines, the first and second marks being formed on the recording paper at at least a scanning-start-side marginal region and a scanning-end-side marginal region in a primary scanning direction perpendicular to the direction of conveyance of the recording paper; and calculating a skew angle using the extents of displacement of the individual test-pattern images.
According to the skew angle calculation method of the present invention, it is possible to calculate a skew angle easily using the test-pattern images that are formed in a simple fashion and in different densities.